This invention relates generally to solid state lasing elements. More particularly, the present invention relates to total internal reflection rod-shaped lasing elements.
Heat is generated in solid-state lasing elements as part of the optical pumping process in which a flashlamp, diode-array, or other sources excite the lasing ions doped into solid-state laser materials such as Nd:YAG, Yb:YAG, Er:YAG, and many others. See D. C. Brown, "Heat, Fluorescence, and Stimulated-Emission Power Densities and Fractions in Nd:YAG," IEEE Journal of Quantum Electronics, Volume 34, pages 560-572, 1998. For very low repetition-rate operation, ample time between pulses allows the laser material to return to thermal equilibrium and no deleterious effects on laser performance occur. When, however, the average pumping and laser output power becomes significant, thermal effects begin to play a significant role and the effect on laser efficiency and operation can be severe.
For rod geometry lasers where the lasing material is a right circular cylinder, attempts to remove the heat generated usually involve cooling the rod 10 along its barrel with water 12 as shown in FIG. 1. Under ideal circumstances in which the pumping of the rod is absolutely uniform and heat is removed in a symmetric and uniform way along the rod barrel, and rod materials parameters independent of temperature, a temperature distribution is established in the rod transverse dimension which can be calculated analytically. It has been found that the radially dependent temperature distribution varies quadratically with the radial coordinate r. See W. Koechner, "Solid-State Laser Engineering," 4th Edition, Springer-Verlag (1996). In essence the heat is distributed across the rod such the highest temperature is at the center of the rod and the lowest temperature is at the rod barrel.
In common laser materials such as Nd:YAG, the index of refraction is a function of temperature and in fact increases with temperature. Hence, there exists a radially varying index of refraction distribution in the rod that follows the temperature distribution wherein the temperature is largest in the center and lowest at the rod edge or barrel. Because of the radially varying index of refraction, light propagation through the rod is affected. Because light travels slower as the refractive index increases (at a speed v=c/n where c is the vacuum speed of light and n the index of refraction), it travels most slowly along the rod axis and fastest at the rod edge.
As shown in FIG. 1, an incident plane wave 14 with flat phase front looks curved 16 after propagating through the rod since the center phase is retarded with respect to the rod edge. It can be shown that ideally the rod functions like a thick lens and that, in the presence of strong thermal effects, the beam exiting the rod is focused. There are two such foci in a laser rod such as Nd:YAG. The tangential and radial polarizations have separate foci that do not overlap, and the focal lengths can be calculated exactly. See W. Koechner, "Solid-State Laser Engineering," 4th Edition, Springer-Verlag (1996). An associated phenomena, birefringence, accompanies strong thermal focusing. For linearly polarized input to the amplifier, a phase difference is accumulated between the radial and tangential components of the polarization. The magnitude of the phase difference depends upon the type of lasing material used, the thermal loading of the rod, and the location in the aperture. Such birefringence is detrimental in rod lasers that must use an intracavity polarizer, such as some Q-switched lasers, since the output from the rod is elliptically polarized and significant losses can result from the polarizer. See W. Koechner, "Solid-State Laser Engineering," 4th Edition, Springer-Verlag (1996).
If the rod is pumped harder and harder the thermally-induced focusing becomes stronger and stronger. Laser resonators that use rod amplifiers are then particularly susceptible to this phenomena. As pumping average power is increased, the rod focuses more strongly. This results in changes in the mode-structure or content of the output beam, a continuous change in the output beam quality, and eventually instability of the resonator thereby causing it to stop lasing. In some resonators, beam quality improves with operating average power until the best beam quality is achieved. Pumping beyond this single point then results in degradation of beam quality.
A number of attempts to reduce or eliminate the thermal focusing have been implemented. For example, a slab 18 (FIG. 2) may be used rather than a rod of laser material such as disclosed in U.S. Pat. No. 3,633,126. The beam is totally-internally-reflected back and forth between the slab faces 20 through which pumping is incident. A flow of water 12 along the slab TIR faces cools the laser crystal (see FIG. 2). The medium used to construct the slab is usually a crystalline or glass material although liquids have also been used. The slab ends 22 are usually either uncoated and cut at or near Brewster's angle so that there are no reflective losses, or anti-reflective (AR) coated for some arbitrary angle of incidence on the faces. The slab is usually thin, typically 5-7 mm, and long. It is usually designed to operate with an even number of bounces off the slab faces through which pump light is delivered to the lasing material. The slab is either transversely cooled with typically water, a water/ethylene glycol mixture, or gas either as shown in FIG. 2 or most often using longitudinal cooling along the slab faces. Conduction cooling has also been used. The pump faces and end faces must be optically flat and accurately cut and oriented; bouncing off the slab faces is via TIR so 100% reflection is obtained.
This technology is referred to in the literature as a face-pumped total-internal-reflection (TIR) laser or slab laser. Other more recent variants of that concept are the hex laser concept of U.S. Pat. No. 4,740,983, and the edge-screw laser of U.S. Pat. No. 4,912,713. Slab laser technology has been developed to the point where kilowatt levels of power can be achieved with excellent output beam-quality that remains constant in the TIR direction. The zig-zag path of the beam back and forth within the laser slab results in internal compensation for thermal effects since each ray incident upon the input aperture experiences approximately the same total thermal environment. Thus, first-order thermal focusing does not occur in a TIR slab laser.
U.S. Pat. Nos. 3,810,040 and 3,810,041 disclose liquid cooled slab lasers. U.S. Pat. No. 3,679,999 discloses a slab laser which is conduction cooled with gas. The main difficulty with the slab laser is that the beam-quality does not remain constant in the direction orthogonal to the TIR direction, or the transverse direction. Many attempts have been made to rectify this situation. However, while the thermal effects may be reduced by various techniques, the output beam-quality and mode-structure remain functions of the average power. When the beam is propagated and used for certain processes, such as percussion drilling, the spot remains constant in size in the TIR direction but varies in the transverse dimension, thereby leading to processing effects that are average power dependent. For many applications this situation is intolerable.
Polygonal rod elements which do not employ TIR to internally self-compensate thermal focusing are disclosed in U.S. Pat. No. 5,432,811. To eliminate the slab's lack of thermal compensation in the transverse direction, "hex" TIR laser elements of U.S. Pat. No. 4,740,983 and square rods of U.S. Pat. No. 4,912,713 totally internally reflect the beam simultaneously in two dimensions. The TIR rod laser element described here has the advantage that it can be used as a replacement for conventional laser rods and remains "in-line" with the resonator optical axis. Conventional laser rods are the most common lasing elements and are pumped typically as shown in FIGS. 3 and 4 where flashlamp and diode-pumping schemes are shown. The rod is in all cases surrounded by a flowtube that encloses an annulus of flowing water or other fluid used to cool the rod.